1. Field of the Invention
The present invention relates to a nitride semiconductor device (for example, InaAlbG1-a-bN, 0≦a, 0≦b, a+b≦1) used as a light emitting device or a light receiving device such as light emitting diode (LED), laser diode (LD), solar cell or light sensor, or other electronic devices such as transistor or power device.
2. Description of the Related Art
Nitride semiconductors are used as the material to make, for example, high luminance blue LED or pure green LED, for such applications as full-color LED display, traffic signal light, image scanner or other light sources. Various LED structures are known including that described in Patent Document 1.
[Patent Document 1]
Japanese Unexamined Patent Publication (Kokai) No. 2000-232237
The light emitting devices of the prior art described above are in some cases insufficient in the device characteristics thereof, such as uneven forward voltage (Vf) within a wafer and variability of Vf with the lapse of driving time. Decreasing the mean value of Vf within a wafer and suppressing the variability in the value of Vf enable it to improve the yield of production. Moreover, suppressing the variability of Vf with the lapse of driving time leads to widening of the applicability of the device and improvement of reliability. In case a plurality of LEDs are connected in parallel, for example, when the value of Vf of only some of the LEDs varies with the drive time, such a problem occurs that the LEDs cannot emit light with even luminance.
Moreover, in a nitride semiconductor device having such a structure as an active layer is sandwiched by an n-type semiconductor layer and a p-type semiconductor layer in order to increase the withstanding electrostatic voltage, providing an n-type semiconductor layer that includes an n-type impurity in the n-type semiconductor layer that comprises a plurality of layers decreases the value of Vf, and providing an n-type semiconductor layer, that is doped with a low concentration of n-type impurity or undoped, improves the withstanding electrostatic voltage. The withstanding electrostatic voltage can be improved further by providing the n-type semiconductor layer, that is doped with a low concentration of n-type impurity or undoped, with a larger thickness, although it makes the problem of variability of Vf with the lapse of driving time more conspicuous.